Method for producing carbon fibers from cellulose fibers

ABSTRACT

A method of producing carbon fibers from cellulose fibers comprises bringing cellulose fibers having a water content of more than 20 parts by weight of water per 100 parts by weight of cellulose fiber into contact with a solution of additives and thereafter converting the additized cellulose fibers comprising not less than 1 part by weight of additives per 100 parts by weight of cellulose into carbon fibers.

The present invention relates to a method of producing carbon fibersfrom cellulose fibers, which comprises bringing cellulose fibers havinga water content of more than 20 parts by weight of water per 100 partsby weight of cellulose fiber into contact with a solution of additivesand thereafter converting the additized cellulose fibers comprising notless than 1 part by weight of additives per 100 parts by weight ofcellulose into carbon fibers.

Carbon fibers are obtainable by pyrolysis of polyacrylonitrile orcellulose fibers. As renewable raw materials, cellulose fibers arebecoming more and more important for the growing market for carbonfibers.

Mingqiu Zhang, S. Zhu, H. Zeng, Y. Lu describe in Die AngewandteMakromolekulare Chemie 222 (1994), 147 -163 (No. 3908) the production ofcarbon fibers from sisal fibers. The sisal fibers are washed with waterand dried. They are then treated with a solution of (NH₄)₂HPO₄ in water,redried and converted into carbon fibers by pyrolysis.

Fanlong Zeng, Ding Pan and Ning Pan use viscose fibers for producingcarbon fibers (Journal of Inorganic and Organometallic Polymers andMaterials, Vol. 15, No. 2 June 2005). Again, dried cellulose fibers aretreated with solutions of additives and then converted into carbonfibers.

Hui Li, Yonggang Yang, Yuefang Wen and Lang Liu (Composites Science andTechnology 67 (2007) 2675-2682) impregnate the viscose fibers with anorganosilicon compound before drying. After the fiber thus pretreatedhas been dried, the cellulose fiber is subjected to a customarytreatment with additives, here an aqueous solution of ammonium sulfateand ammonium chloride, and finally carbonized.

Cellulose dissolved in ionic liquid is also used in CN 101871140 forproduction of carbon fibers.

Processes for producing carbon fibers should have a very high carbonyield; that is, ideally all of the carbon in the starting fiber shouldend up in the carbon fiber. The carbon yield is still unsatisfactorywhen cellulose fibers are used. Some of the cellulose carbon is lost bydegradation into ultimately carbon monoxide and carbon dioxide. There isalso a need to further improve the mechanical properties of the carbonfibers obtained from cellulose fibers.

The problem addressed by the present invention was therefore that ofproviding an improved method of producing carbon fibers from cellulosefibers.

The problem was solved by the method defined at the outset.

The Cellulose Fibers

The cellulose fibers are the starting material for the method. Cellulosefibers within the meaning of the present invention are fibers consistingof cellulose or modified cellulose to an extent of more than 60 wt %,particularly more than 80 wt %, more preferably more than 90 wt %.

In one particular embodiment, the cellulose fibers consist of celluloseor modified cellulose to an extent of more than 98 wt %, most preferably100 wt %.

Modified cellulose is cellulose with etherified or esterified hydroxylgroups. Modified cellulose may comprise for example cellulose acetate,cellulose formate, cellulose propionate, cellulose carbamate orcellulose allophanate.

The cellulose fibers used preferably consist of cellulose in theabove-specified minimum amounts.

The cellulose fibers brought into contact with a solution of an additivehave a water content of more than 20 parts by weight of water,particularly more than 30 parts by weight of water, more preferably morethan 50 parts by weight of water and most preferably more than 70 partsby weight of water per 100 parts by weight of cellulose fiber.

In general, however, the water content is not higher than 500,particularly not higher than 300 parts by weight of water per 100 partsby weight of cellulose fiber.

The cellulose fiber having the above water content is obtainable in asimple manner, for example by dipping a dried cellulose fiber intowater. Not only natural cellulose fibers are suitable for this purpose,but also synthetic cellulose fibers.

Natural cellulose fibers are in particular cellulose fibers derived fromcotton.

A preferred embodiment uses synthetic cellulose fibers.

A preferred embodiment uses synthetic cellulose fibers obtainedimmediately beforehand via a spinning process.

The cellulose fibers are then preferably obtained by

-   -   spinning the cellulose fibers from a spinning solution    -   and then washing the cellulose fibers with water.

In the above spinning process, a spin bath is prepared by dissolvingcellulose in a solvent. The cellulose fiber is derived from this spinbath by coagulating the cellulose in the form of a fiber.

There are various species of cellulose fibers depending on solvent andadjuncts used in the spin bath:

viscose fibers produced by the viscose process,

Lyocell® fibers produced from a spinning solution comprising NMMO(N-methylmorpholine N-oxide) as solvent, or

cellulose fibers obtained from spinning solutions comprising ionicliquid as solvent, as described in WO 2007/076979 for example.

In all the cases above, the cellulose fibers obtained are washed withwater in order to remove adherent solvent or adherent additives from thespin bath.

The step of contacting with water is effected such that the cellulosefiber takes up water in the desired amount specified above. To this end,the cellulose fiber may be dipped into water for a sufficient length oftime or, in a continuous process, be led through a sufficiently longwater bath.

Preferably, the process for producing the cellulose fibers does notinvolve any measures for drying. The cellulose fiber obtained in thespinning process is washed with water without previous drying andthereafter, again of course without previous drying, brought intocontact with the solution of the additive. The cellulose fiber concernedis therefore known as a never-dried cellulose fiber.

The Additization of Cellulose Fibers

The aqueous cellulose fibers are brought into contact with a solution ofadditives.

The solution of additives is preferably a solution of additives in ahydrophilic solvent, particularly in water, hydrophilic organicsolvents, e.g., alcohols or ethers, or mixtures thereof. Particularpreference for use as hydrophilic solvents is given to water or mixturesof water with other hydrophilic organic solvents miscible with water inany proportion, while in the last case, in a preferred embodiment, thewater fraction in the solvent mixture is not less than 50 wt %.

A solution of additives in water is concerned in particular.

The solution may comprise just a single additive or a mixture of variousadditives.

Useful additives include particularly compounds having a watersolubility of not less than 10 parts by weight, preferably of not lessthan 20 parts by weight, in particular of not less than 30 parts byweight, per 100 parts by weight of water under standard conditions (20°C., 1 bar).

The additives preferably comprise low molecular weight compounds havinga molecular weight of not more than 1000 g/mol, more preferably not morethan 500 g/mol, particularly not more than 300 g/mol.

Possible additives in one preferred embodiment include salts or acids,e.g., inorganic salts, inorganic acids, organic salts or organic acids.

Suitable organic acids are, in particular, carboxylic acids, sulfonicacids or phosphonic acids.

Suitable organic salts are, in particular, salts of the foregoingorganic acids, in which case metal salts may be concerned, in particularalkali metal salts, or salts having organic cations.

Suitable organic acids include, for example, those which react with thecellulose and become attached thereto through a chemical reaction, forexample a substitution reaction.

Phosphoric acid is particularly useful as inorganic acid.

Suitable inorganic salts are particularly those whose anions comprisephosphorus atoms, sulfur atoms, nitrogen atoms, for example in the formof phosphate, hydrogenphosphate, phosphite, hydrogenphosphite, sulfateor sulfite, or comprising chloride as anion.

The cations of the above inorganic salts may comprise particularly metalcations, preferably alkali metal cations such as Na⁺ or K⁺, or ammonium(NH₄ ⁺).

(NH₄)₂HPO₄, NH₄SO₄ or NH₄Cl may be mentioned by way of example.

The above additives are frequently additives also used as flameretardants. It is believed that these additives interact with theprimary hydroxyl group on the glucose ring (i.e., the CH₂OH group) andwork during the pyrolysis to counteract the cellulose breaking down intovolatile compounds of carbon.

The total amount of all additives in the solution is, for example, from0.05 to 5 mol/per liter of solution, preferably from 0.1 mol to 2mol/per liter of solution.

The step of contacting with the solution of the additives is effectedsuch that the cellulose fiber takes up additives in the desired amount.To this end, the cellulose fiber may be dipped into the solution for asufficient length of time or, in a continuous process, be led through asufficiently long solution bath.

In one preferred embodiment, the cellulose fiber is led through thesolution of the additive in a continuous manner.

The contact time of the cellulose fiber with the solution of theadditives is preferably not less than 0.5 second, more preferably notless than 2 and most preferably not less than 10 seconds. The contacttime is generally not longer than 100 seconds, preferably not longerthan 30 seconds.

The additized cellulose fiber obtained comprises not less than 5 partsby weight of additives per 100 parts by weight of cellulose in apreferred embodiment. The additized cellulose fiber more preferablycomprises not less than 1 part by weight of additives and mostpreferably not less than 3 parts by weight of additives per 100 parts byweight of cellulose fiber. The cellulose fiber generally comprises notmore than 30 parts by weight of additives, particularly not more than 10and/or not more than 5 parts by weight of additives per 100 parts byweight of cellulose fiber.

The production of the cellulose fiber in a spinning process and thesubsequent further processing by washing the cellulose fiber andcontacting the cellulose fiber with the solution of the additives arepreferably constituent parts of a continuous overall process. In it,once the cellulose fiber has been obtained, it is generally fed into theindividual further processing steps via mobile rollers.

Finally, excess solvent from the solution of the additives may beremoved by squeezing and the additized cellulose fiber may be rolled up.

The additized cellulose fiber may finally be dried, for example attemperatures of 50 to 300° C. Such drying is advisable when theadditized cellulose fiber is first to be stored or transported beforeits conversion into a carbon fiber.

Lastly, the additized cellulose fiber is converted into a carbon fiberby pyrolysis.

The pyrolysis is generally carried out at temperatures of 500 to 1600°C. It may for example be carried out under air or under a protectivegas, for example nitrogen or helium. It is preferably carried out undera protective gas.

The cellulose fiber may be dried before pyrolysis. In the case ofpreviously dried and stored cellulose fibers, drying may optionally berepeated.

One possibility is a multi-step process wherein the cellulose fiber isdried at temperatures in the range from 50 to 300° C. and then pyrolyzedat temperatures in the range from 500 to 1600° C., preferably from 700to 1500° C.

Both the drying temperature and the pyrolysis temperature may be raisedstepwise or continuously.

One possibility, for example, is a drying operation in two or moresteps, for example at 50 to 100° C. in a first step and at 100 to 200°C. in a second step. Contact time in the individual steps may be forexample from 5 to 300 seconds in each case and range in total from 10 to500 seconds during drying.

One possibility, for example, is a pyrolysis where the temperature israised continuously, for example from 200° C. at the beginning until theeventual attainment of 1600 or 1400 or 1200° C. The rate of temperatureincrease may be from 1 to 20 kelvins/minute for example.

Exposure of the cellulose fiber to a temperature in the range from 900to 1600° C. should preferably be for 10 to 60 minutes.

The carbon yield at pyrolysis is generally from 20 to 95 wt %; that is,the carbon fiber comprises from 20 to 95 weight percent of the carbonpresent in the cellulose fiber. The carbon yield is more particularlyfrom 70 to 95, more preferably from 70 to 90 and most preferably from 70to 85 wt %.

The process of the present invention provides an enhanced carbon yield.The carbon fiber obtained has very good mechanical properties, inparticular good tenacity and elasticity.

EXAMPLES

Cellulose Fiber

The cellulose fiber employed in the example of the invention and in thecomparative example is a synthetic high-tenacity cellulose fiber of thetype used in the manufacture of automotive tires. Cellulose fibers ofthis type are known as tire cord fibers. The cellulose fiber in theexample of the invention was never dried after its formation, hence itsappellation as “never-dried tire cord fiber”. The cellulose fiber in thecomparative example was dried. However, cellulose fibers typicallycomprise bound residual water, so the water content of dried cellulosefibers may be, for example, up to 20 wt %.

Example 1

The never-dried tire cord fiber presented for additization has a watercontent of 150% and numbers 1000 filaments having a linear density of2.2 dtex per filament.

The fiber is additized and dried in a continuous process on godets.Godets are rolls transporting the fiber in a continuous manner throughthe plant. Four (4) of these godets are employed. Between the first andsecond godets, the fiber is loaded with the additives via a dip bath.Between the third and fourth godets, a hot air duct effects a dryingoperation. At the end, a tension-controlled winder winds up theadditized and dried fibrous material.

The godets all run at a speed of 6 m/min. The first godet serves to haulup the water-stored never-dried tire cord fibers. The fiber is wrappedaround the godet 2 times, which corresponds to a contact time of 10 sec.The fiber is then directed through a dip bath containing an ammoniumdihydrogenphosphate solution (concentration of ammoniumhydrogenphosphate: 0.54 mol/l). The residence time therein amounts toabout one second. The material is then wrapped six times around thesecond godet. This step is designed to allow surplus additization todrip off and to effect a homogeneous distribution of the ammoniumhydrogen-phosphate in the fiber. The contact time here is 72 seconds.This is followed by drying on the heated third godet at 80° C. Here 10wraps correspond to a contact time of 100 sec. The fiber is then ledthrough a hot-air duct at 150° C. The residence time therein is 12 sec.The dry strand is then wrapped 4 times around the last godet (contacttime 24 sec) before a tension-controlled winder winds up the material ata pre-tension of 0.1 cN/tex.

The additized cellulose fiber is subsequently carbonized in two stagesunder a protective gas. In the first stage, the fiber is heated at 2°K/min to 260° C. and after a residence time of 10 min heated at 10 k/minto 1400° C. and thereafter cooled down.

The carbon yield is 80 wt %, the fiber tenacity is 1.4 Gpa and theelongation at break is 3.1%.

Comparative Example with Drying Prior to Additization

The dried tire cord fiber presented for additization has a water contentbelow 20 wt % and numbers 1000 filaments having a linear density of 2.2dtex per filament. The experimental procedure corresponds to Example 1.

The carbon yield is 65 wt %, the fiber tenacity is 1.1 Gpa and theelongation at break is 2.2%.

1. A method of producing carbon fibers from cellulose fibers,comprising: contacting cellulose fibers comprising more than 20 parts byweight of water per 100 parts by weight of cellulose fiber with asolution comprising an additive such that additized cellulose fiberscomprising not less than 1 part by weight of an additive per 100 partsby weight of cellulose are produced; and converting the additizedcellulose fibers into carbon fibers.
 2. The method according to claim 1,wherein the cellulose fibers comprise more than 50 parts by weight ofwater per 100 parts by weight of cellulose.
 3. The method according toclaim 1, wherein the cellulose fibers are obtained by the processcomprising spinning raw material cellulose fibers from a spinningsolution, and washing the spun raw material cellulose fibers with water.4. The method according to claim 3, wherein the process does notcomprise drying the raw material cellulose fibers or the spun rawmaterial cellulose.
 5. The method according to claim 1, wherein thesolution comprises water, a hydrophilic organic solvent or mixturesthereof.
 6. The method according to claim 1, wherein the additivecomprises a compound having a water solubility of not less than 10 partsby weight per 100 parts by weight of water under standard conditions(20° C., 1 bar).
 7. The method according to claim 1, wherein theadditive comprises a salt or an acid.
 8. The method according to claim1, wherein the contacting comprises leading the cellulose fibers throughthe solution in a continuous manner such that a contact time is not lessthan 2 seconds.
 9. The method according to claim 1, wherein theadditized cellulose fibers comprise not less than 5 parts by weight ofthe additive per 100 parts by weight of cellulose.
 10. The methodaccording to claim 1, wherein the additized cellulose fibers areconverted into a the carbon fibers by pyrolysis.
 11. The methodaccording to claim 1, further comprising: producing the cellulose fibersby a process comprising spinning raw material cellulose fibers from aspinning solution, and washing the spun raw material cellulose fiberswith water.
 12. The method according to claim 11, wherein the processfor producing the cellulose fibers does not comprise drying the rawmaterial cellulose fibers or the spun raw material cellulose.
 13. Themethod according to claim 10, wherein the pyrolysis is performed at atemperature of from 500 to 1600° C.
 14. The method according to claim10, wherein the pyrolysis is performed by heating the additizedcellulose fibers while increasing a temperature from 200° C. to 1200° C.15. The method according to claim 1, further comprising: drying thecarbon fibers at a temperature of from 50 to 300° C.